Method of producing oxide ion conductor

ABSTRACT

An La 2 O 3  powder and an SiO 2  powder are mixed with each other, and then heated. By heating, a porous material of La X Si 6 O 1.5X+12  (8≦X≦10) as a composite oxide is produced. Subsequently, the porous material is pulverized to obtain a powder, and the powder is added to a solvent to prepare a slurry. The slurry is solidified in a magnetic field to prepare a compact. After that, the compact is sintered, and an oxide ion conductor is obtained thereby.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide ion conductor and a method ofproducing the same. In particular, the present invention relates to anoxide ion conductor which is composed of a composite oxide that exhibitsanisotropy in relation to the oxide ion conductivity. The oxide ionconductor is preferably usable as a solid electrolyte of a fuel cell.The present invention also relates to a method of producing the oxideion conductor.

2. Description of the Related Art

An oxide ion (O²⁻) conductor is suggested as an electrolyte of the fuelcell. The fuel cell attracts the attention as a low pollution electricpower supply source in response to the growing concern about theenvironment protection in recent years. When the oxide ion conductor isused, the entire fuel cell can be made of solid materials, because theoxide ion conductor is a solid. Therefore, the structure is simple.Further, it is possible to reduce the frequency of the maintenanceoperation because no liquid leakage occurs.

Thus, Japanese Laid-Open Patent Publication Nos. 8-208333 and 11-130595suggest oxide ion conductors composed of a composite oxide of a rareearth element and Si. On the other hand, the present applicant-hassuggested, in Japanese Laid-Open Patent Publication No. 2002-252005, anoxide ion conductor which is composed of a composite oxide of a rareearth element and Ge and which can be manufactured at a low temperatureas compared with the oxide ion conductors as described above. The oxideion conductor as described above is manufactured by sintering a powderof the oxide of the rare earth element and a powder of silicon oxide orgermanium oxide. The oxide ion conductor obtained in this way is thecomposite oxide, and the crystalline structure thereof is an apatitetype structure.

It is preferable that the oxide ion conductor of this type is excellentin oxide ion conductivity. However, it is not easy to remarkably improvethe oxide ion conduction. In particular, when it is intended tomanufacture single crystals by a method described in Japanese Laid-OpenPatent Publication No. 11-130595, a shape of the single crystal isspecified to certain ones. Further, it is necessary to melt thematerial. Therefore, the oxide ion conductor, which is finally obtained,has a narrow composition range.

The present inventors note the mechanism of the oxide ion conduction inthe composite oxide during the study for improving the oxide ionconductivity of the composite oxide having the apatite type structure.

The crystal system of the substance having the apatite type structureusually belongs to the hexagonal system. In this case, the oxide ionconduction is caused by the movement of an oxide ion (O²⁻) present atthe 2 a site. Accordingly, it can be assumed that the oxide ionconductivity, which is obtained in the movement direction, may beimproved if the directions of the movement of O²⁻ are approximatelycoincident with each other in the respective crystal grains by aligningthe directions of orientation of the crystal grains of the compositeoxide. In other words, it is considered that anisotropy is brought aboutin the directions of orientation of the crystals and consequently in theoxide ion conductivity so that ions are moved in the direction in whichthe oxide ion conductivity is high. However, an oxide ion conductorhaving the apatite type structure has not been found, in which thedirections of orientation of the crystals are approximately coincidentwith each other. Also, the method of producing such an oxide ionconductor has not been found.

Each of Japanese Laid-Open Patent Publication Nos. 2002-53367 and2002-193672 suggests a ceramic sintered product having anisotropy and amethod of producing the same. A powder is dispersed in a solvent toprepare a slurry, and then the slurry is solidified in a magnetic fieldto obtain a compact which is thereafter sintered. However, such atechnique is used to improve the mechanical characteristics such asstrength and toughness in a specified direction, which is not directedto the improvement in the oxide ion conductivity in a specifieddirection of a composite oxide having the apatite type structure or thelike.

SUMMARY OF THE INVENTION

The present inventors have studied the improvement in oxide ionconductivity. As a result, the present inventors have found out that thedirections of orientation of crystals can be approximately aligned.

A principal object of the present invention is to provide an oxide ionconductor and a method of producing the same. The oxide ion conductorhas anisotropy in relation to a movement direction of oxide ion becauseof the approximate coincidence of orientations of crystal grains. Theoxide ion conductor is extremely excellent in oxide ion conductivity ina specified direction.

According to a first aspect of the present invention, there is providedan oxide ion conductor in which oxide ion conductivity is shown in adirection of conduction or on a plane of conduction for moving oxide ionin a crystal, wherein the oxide ion conductor has anisotropy in theoxide ion conductivity.

In the oxide ion conductor having the anisotropy in the oxide ionconductivity, the oxide ion conductivity is extremely increased in thedirection or in the plane in which the oxide ion is easily movable. Inother words, the satisfactory oxide ion conductor is provided.Therefore, the oxide ion conductor is preferably usable as a conductorfor which large oxide ion conductivity is required, for example, in anelectrolyte of a solid oxide fuel cell.

Preferred examples of the oxide ion conductor as described above mayinclude a composite oxide which has constituent elements of a trivalentelement A, a tetravalent element B, and oxygen O, the composite oxidehas a composition formula represented by A_(X)B₆O_(1.5X+12) providedthat 8≦X≦10, the composite oxide has an apatite type crystallinestructure, and

the oxide ion conductor has anisotropy in the oxide ion conductivity.

In the oxide ion conductor, the orientation of each crystal of the oxideion conductor is approximately a specified direction. In other words,the orientations of the crystals are approximately aligned. Therefore,the directions, in which O²⁻ is moved in the crystals, are approximatelycoincident with each other. Accordingly, the oxide ion conductivity isimproved in the movement direction.

It is preferable that the crystal system of the composite oxide has ahexagonal system, and the direction of crystals is oriented in a c-axis.

In general, it is considered that the oxide ion conduction is providedas O²⁻ is moved along the c-axis in the apatite type structure belongingto the hexagonal system. Therefore, when the crystals are oriented inthe c-axis direction, it is possible to provide the oxide ion conductorwhich is extremely excellent in the oxide ion conductivity in the c-axisdirection in which O²⁻ is considered to be moved.

It is especially preferable that the crystal system belongs to thehexagonal system, and a space group of the crystal is expressed as P6₃/mby a Hermann-Mauguin's symbol. When the crystal system as describedabove is formed, the oxide ion conductivity is increased maximally.

Preferred examples of the element A may include a rare earth element. Lais especially preferred. On the other hand, preferred examples of theelement B may include Si and Ge.

Other preferred examples of the oxide ion conductor in which theconduction direction or the conduction plane for moving the oxide ionexists in the crystal may include layered perovskite compounds and aseries of oxide ion conductors referred to as “BIMEVOX” having a basiccomposition of Bi₄V₂O₁₁. The oxide ion conductor according to thepresent invention also includes the compounds as described aboveprovided that the compounds have the anisotropy in the oxide ionconductivity.

According to a second aspect of the present invention, there is provideda method of producing an oxide ion conductor comprising a compositeoxide which has constituent elements of a trivalent element A, atetravalent element B, and oxygen O, the composite oxide has acomposition formula represented by A_(X)B₆O_(1.5X+12) provided that8≦X≦10, the composite oxide has an apatite type crystalline structure,the oxide ion conductor has anisotropy in oxide ion conductivity, themethod of producing the oxide ion conductor comprising:

a first step of mixing a powder of a substance having constituentelements of the element A and oxygen O with a powder of a substancehaving constituent elements of the element B and oxygen O in a ratio toproduce A_(X)B₆O_(1.5X+12) provided that 8≦X≦10, for obtaining a mixed

a second step of heating the mixed powder for preparing the compositeoxide having the composition formula represented by A_(X)B₆O_(1.5X+12)provided that 8≦X≦10;

a third step of adding the composite oxide to a solvent for preparing aslurry, and then solidifying the slurry in a magnetic field forpreparing a compact; and

a fourth step of sintering the compact for preparing the oxide ionconductor of the composite oxide.

When the slurry is arranged in the magnetic field, the crystal grains ofthe composite oxide contained in the slurry are approximately oriented.Accordingly, the compact is obtained, in which the crystal grains areapproximately oriented in a specified direction. Consequently, it ispossible to obtain the sintered product which exhibits the anisotropy inthe oxide ion conductivity.

It is preferable that a temperature of heating in the second step is700° to 1200° C. If the temperature is less than 700° C., the heating isnot sufficient. On the other hand, if the temperature exceeds 1200° C.,the grain growth is excessively advanced, and a relatively densesintered product may be obtained. In such a case, it is not easy toprepare the slurry in the next third step.

It is preferable that a temperature of sintering in the fourth step is1400° to 1800° C. As described above, according to the presentinvention, the sintering temperature can be not more than 1800° C.Therefore, it is possible to realize a long service life of a reactionfurnace and prevent decomposition of the oxide ion conductor. Inparticular, when Ge is selected as the element B, the sinteringtemperature can be a relatively low temperature, i.e., about 1500° C.Thus, it is possible to reduce the production cost of the oxide ionconductor. If the temperature is less than 1400° C., the grain growth isnot advanced efficiently.

Preferred examples of the substance having the constituent elements ofthe element A and oxygen O may include rare earth compounds, especiallylanthanum compounds such as lanthanum oxide (La₂O₃), lanthanum hydroxide(La(OH)₃), and lanthanum carbonate (La₂CO₃). Among them, lanthanum oxideis most preferred.

On the other hand, preferred examples of the substance having theconstituent elements of the element B and oxygen O may include siliconoxide and germanium oxide.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view illustrating an entire oxide ionconductor according to an embodiment of the present invention;

FIG. 2 shows a schematic structure of a unit lattice ofLa_(X)Si₆O_(1.5X+12) (8≦X≦10) of the oxide ion conductor shown in FIG.1;

FIG. 3 shows a graph illustrating relationships between the currentdensity, the output density and the electric potential at 700° C. inrelation to a c-axis-parallel material and a c-axis-perpendicularmaterial;

FIG. 4 shows a graph illustrating relationships between the currentdensity, the output density and the electric potential at 800° C. inrelation to the c-axis-parallel material and the c-axis-perpendicularmaterial;

FIG. 5 is an illustrative view of a magnified major part illustratingthe c-axis directions of respective crystals in a general oxide ionconductor;

FIG. 6 is an illustrative view of a magnified major part illustratingthe c-axis directions of respective crystals in the oxide ion conductorshown in FIG. 1;

FIG. 7 shows a flow chart illustrating a method of producing the oxideion conductor according to an embodiment of the present invention;

FIG. 8 shows an X-ray diffraction measurement profile of the oxide ionconductor (measured in the c-axis direction) according to the embodimentof the present invention in which a compact was manufactured in thepresence of a magnetic field;

FIG. 9 shows an X-ray diffraction measurement profile of a conventionaloxide ion conductor in which a compact was manufactured without anymagnetic field; and

FIG. 10 shows a graph illustrating comparison of ion conductivities ofthe oxide ion conductor in the c-axis direction shown in FIG. 8, theoxide ion conductor shown in FIG. 9, and yttria-stabilized zirconia(YSZ).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The oxide ion conductor of the present invention and the method ofproducing the same will be explained in detail below with reference tothe accompanying drawings as exemplified by preferred embodiments.

An oxide ion conductor according to an embodiment of the presentinvention is shown in FIG. 1. The oxide ion conductor 10 is a sheet-likesintered product composed of a composite oxide having the element A andthe element B. The composition formula thereof is represented byA_(X)B₆O_(1.5X+12).

In this formula, X is within a range of not less than 8 and not morethan 10. If X is less than 8, it may be difficult for the crystallinestructure to form the apatite type structure. In other cases, anyimpurity phase such as La₂Si₂O₇ may be contained. On the other hand, ifX exceeds 10, any impurity phase such as La₂SiO₅ may be contained. Inany case, the oxide ion conductivity may be lowered.

A trivalent element is selected for the element A, and a tetravalentelement is selected for the element B. In particular, when X is not lessthan 8 and not more than 10, then the apatite type structure is formed,and the excellent oxide ion conductivity is obtained. Therefore, a rareearth element, especially La is preferred for the element A, and Si orGe is preferred for the element B. The range of X is more preferably notless than 9 and not more than 10. On this condition, it is possible toreliably obtain the crystals having the apatite type structure. When Geis selected for the element B, it is preferable that X is not less than8 and less than 10.

The most preferable value of X is 9.33. In this case, the crystals formthe apatite type structure (see FIG. 2), and an impurity phase havingany other structure is scarcely observed. That is, the oxide ionconductivity is maximized.

FIG. 2 shows the structure of the unit lattice of La_(X)Si₆O_(1.5X+12)(8≦X≦10) as the composite oxide as viewed in the c-axis direction. Theunit lattice 11 has the apatite type structure including six SiO₄tetrahedrons 12, O²⁻ 14 occupying the 2 a sites, and La³⁺ 16 a, 16 boccupying the 4 f sites or the 6 h sites, respectively. Si⁴⁺ and O²⁻ ofthe SiO₄ tetrahedron 12 are not shown.

The crystal system of the unit lattice 11 belongs to the hexagonalsystem. That is, in FIG. 2, the angle α at which the edge AB in thea-axis direction and the edge BF in the c-axis direction of the unitlattice 11 intersect with each other, the angle β at which the edge BCin the b-axis direction and the edge BF intersect with each other, andthe angle γ at which the edge AB and the edge BC intersect with eachother are 90°, 90°, and 120°, respectively. The length of the edge AB isequal to the length of the edge BC. Further, the length of the edges AB,BC is longer than the length of the edge BF.

In the hexagonal system lattice, when the lattice is rotated by ⅓ turnabout the center of the virtual screw axis (not shown) and is translatedby ½ of the length of the edge BF along the screw axis, the positions ofthe respective ions are coincident with each other before and after theoperation. Further, the mirror plane of the hexagonal system lattice isperpendicular to the screw axis. That is, the space group of the crystalof La_(X)Si₆O_(1.5X+12) (8≦X≦10) is expressed as P6₃/m according to aHermann-Mauguin's symbol.

The oxide ion conductor 10 according to the embodiment of the presentinvention is the sintered product in which crystal grains having thecrystalline structure as described above are sintered with each other,in which the c-axis is oriented in the direction of the arrow B.Therefore, the oxide ion conductivity in the direction of the arrow B(thickness) is higher than the oxide ion conductivity in the directionof the arrow A shown in FIG. 1 in the oxide ion conductor 10. That is,the oxide ion conductor 10 has the anisotropy in relation to the oxideion conductivity.

It is considered that the reason, why the anisotropy occurs in the oxideion conductivity of the oxide ion conductor 10, is as follows.

O²⁻ 14, which occupies the 2 a sites in La_(X)Si₆O_(1.5X+12) (8≦X≦10)having the apatite type structure shown in FIG. 2, does not participateso much in the bonding with respect to the SiO₄ tetrahedrons 12 or La³⁺16 a. Accordingly, the force, which acts on O²⁻ 14, is not strong.Therefore, it is expected that O²⁻ 14 can be moved relatively freely inthe c-axis direction without being restricted by the 2 a sites. It isconsidered that the oxide ion conduction is caused by the movement ofthe oxide ion based on the mechanism as described above.

When samples are actually cut out from a portion extending in theparallel direction along the c-axis and a portion extending in thedirection perpendicular to the c-axis from a single crystal manufacturedby the Bernoulli method, the Czochralski method (respective samples willbe hereinafter referred to as “c-axis-parallel material” and“c-axis-perpendicular material”) or the like, and electriccharacteristics are investigated for the c-axis-parallel material andthe c-axis-perpendicular material, the c-axis-parallel material issuperior.

Specifically, when fuel cells are produced by using the c-axis-parallelmaterial and the c-axis-perpendicular material as an electrolyte, andwhen the output density with respect to the current density is measuredat 700° C. or 800° C. for the c-axis-parallel material and thec-axis-perpendicular material, the output density of the c-axis-parallelmaterial is extremely larger than that of the c-axis-perpendicularmaterial in any case as shown in FIGS. 3 and 4.

When the electric potential with respect to the current density ismeasured, a high electric potential is obtained for the c-axis-parallelmaterial even when the current density is large as shown in FIGS. 3 and4 as well. This means the fact that high voltage is obtained, forexample, when the c-axis-parallel material is used as an electrolyte ofa fuel cell and the fuel cell is subjected to electric discharge at alarge current density.

However, when the single crystal is manufactured by the Bernoullimethod, the Czochralski method or the like, it is difficult tomanufacture products having shapes other than columnar shapes, and it isdifficult to manufacture a large crystal. Further, only the oxide ionconductor, in which the composition range is relatively narrow, can beobtained, because the material is melted. In particular, when the oxideion conductor, which contains Ge as the element B, is manufactured, thecomposition of Ge is changed during the melting, because the meltedmatter has a high vapor pressure. For this reason, it is difficult tomanufacture the single crystal.

In the case of a general sintered product, as shown in FIG. 5, theorientation of the c-axis, in other words, the orientation of thecrystal grains is irregular. Therefore, the oxide ion conductivity ofsuch a sintered product exhibits the isotropy.

In contrast, in the case of the oxide ion conductor 10 according to theembodiment of the present invention, as shown in FIG. 6, the c-axis ofthe respective crystal grains is approximately directed in the samedirection. That is, the orientations of the crystal grains areapproximately aligned. Accordingly, the direction of movement of theoxide ions along the c-axis is aligned in each of the crystal grains.Therefore, the oxide ion conductivity is extremely large in thedirection along the c-axis (direction of the arrow B shown in FIG. 1).On the other hand, the movement of the oxide ion is scarcely caused inthe directions other than the direction parallel to the c-axis.Therefore, the oxide ion conductivity is low in the directions otherthan the direction parallel to the c-axis. It is considered that theanisotropy is consequently generated in the oxide ion conductivity.

As described above, the oxide ion conductor according to the embodimentof the present invention exhibits the excellent oxide ion conductivityin the direction along the c-axis. Therefore, as compared with aconventional fuel cell which has a conventional oxide ion conductor, afuel cell which uses the oxide ion conductor according to the embodimentof the present invention as a solid electrolyte shows equivalent powergeneration characteristics even when the operation is performed at a lowtemperature. Therefore, it is possible to reduce the operation cost ofthe fuel cell.

The oxide ion conductor 10, in which the crystal grains areapproximately oriented in the specified direction, can be produced asfollows.

A method of producing the oxide ion conductor 10 according to theembodiment of the present invention will be explained with reference toa flow chart of FIG. 7. In this example, La_(X)Si₆O_(1.5X+12) isproduced while selecting La as the element A and selecting Si as theelement B. This production method comprises a first step S1 of mixing alanthanum oxide powder and a silicon oxide powder to prepare a mixedpowder, a second step S2 of applying a heat treatment to the mixedpowder, a third step S3 of adding a composite oxide produced by the heattreatment to a solvent to prepare a slurry and solidifying the slurry ina magnetic field to prepare a compact, and a fourth step S4 of sinteringthe compact to prepare a sintered product (oxide ion conductor).

In the first step S1, the lanthanum oxide (La₂O₃) powder and the siliconoxide (SiO₂) powder are mixed with each other.

In this step, the ratio between the La₂O₃ powder and the SiO₂ powder isset so that the crystal of La_(X)Si₆O_(1.5X+12) as a final product hasthe apatite type structure, in other words, the value of X is not lessthan 8 and not more than 10. For example, to obtain a composite oxidehaving a composition represented by La_(9.33)Si₆O₂₆, the ratio La₂O₃powder: SiO₂ powder=4.22:1 (numerals are indicated by weight ratios).

When the powders are mixed, a known technique such as a wet ball millmethod may be adopted while adding ethanol or the like.

Subsequently, in the second step S2, the mixed powder is heat-treated.La₂O₃ and SiO₂ are reacted with each other in accordance with the heattreatment. Accordingly, La_(X)Si₆O_(1.5X+12) (8≦X≦10) as the compositeoxide is produced.

During this process, the heat treatment is conducted at a temperature atwhich the grain growth of La_(X)Si₆O_(1.5X+12) is not excessivelyadvanced and a porous material, which is easily pulverized underpressure, can be obtained. Specifically, it is preferable that thetemperature is 700° to 1200° C. The heat treatment time may be, forexample, about 2 hours.

The obtained porous material as described above is pulverized to preparea powder. Subsequently, the compact is manufactured by using the powderin the third step S3.

The powder is added to the solvent to prepare the slurry. An example ofthe solvent is ethanol. The amount of the solvent may be an amount sothat the ratio of the powder is about 40% by volume. A dispersing agentsuch as SN Dispersant 9228 (trade name of the product by San NopcoLimited) may be further added to the slurry. It is sufficient to add thedispersing agent thereto about 2.5% by volume.

It is preferable that the wet ball mill method is applied to the slurry.Accordingly, the grain sizes of powder may be further decreased, andgrain growth is promoted in the powder during the sintering. Further,the powder is dispersed substantially uniformly in the slurry.Therefore, it is possible to obtain a dense and strong sintered product.

Subsequently, the slurry is formed to a sheet-shaped. That is, apredetermined amount of the slurry is put into a rectangular frame. Inthe embodiment of the present invention, the slurry is settled in themagnetic field together with the frame.

As shown in FIG. 2, in the crystal having the apatite type structure,the length of the edge AB (in the a-axis direction) is equal to thelength of the edge BC (in the b-axis direction), and the edges AB, BCare longer than the edge BF (in the c-axis direction). Therefore, it isexpected that the magnetic susceptibilities in the a-axis direction andin the b-axis direction are different from the magnetic susceptibilityin the c-axis direction.

Therefore, each crystal grain is approximately oriented in the magneticfield so that the c-axis direction is parallel or perpendicular to thedirection of lines of magnetic force. As a result, the compact isobtained, in which each crystal grain is approximately oriented in thespecified direction.

The intensity of the magnetic field may be about 10 T (tesla). Theslurry may be left until the slurry is solidified to form a compact. Inthe slurry in which the ratio of the powder is 40% by volume, the slurryis solidified in about 6 hours to form the compact.

Subsequently, in the fourth step S4, the grain of La_(X)Si₆O_(1.5X+12)powder grows by sintering the compact. That is, the joined portions ofthe grains grows, at which the grains contact each other, and the grainsare finally jointed to one another to form large grains. Accordingly,the oxide ion-conductor 10 as the sintered product (see FIG. 1) isconsequently obtained.

It is preferable that the sintering temperature is 1400° to 1800° C. Ifthe sintering temperature is less than 1400° C., the grain growth is notadvanced efficiently. On the other hand, if the temperature exceeds1800° C., the sintered product does not have a desired composition orthe apatite type structure by thermal decomposition in sintering. Thus,the oxide ion conductivity is undesirably lowered. When Si is selectedfor the element B, the sintering temperature is more preferably 1450° to1700° C., and the sintering temperature is much more preferably 1700° C.When Ge is used for the element B, the sintering temperature of 1500° C.is preferable.

As described above, in the production method according to the embodimentof the present invention, it is possible to lower the sinteringtemperature. Therefore, it is possible to realize a long service life ofthe reaction furnace. Further, it is also possible to reduce theproduction cost.

The sintering time is chosen as only a period of time in which the densesintered product (oxide ion conductor 10) is obtained. For example, whenthe sintering temperature is 1500° C., the temperature is maintained forabout 6 hours.

FIG. 8 shows an X-ray diffraction measurement profile of the oxide ionconductor 10 (having the composition of La₁₀Si₆O₂₇) obtained by theproduction method as described above. The oxide ion conductor 10 ismeasured in the c-axis direction. FIG. 9 shows an X-ray diffractionmeasurement profile of La₁₀Si₆O₂₇ produced in accordance with thisproduction method except that a compact was manufactured in no magneticfield. When the respective peak intensities on the (002) and (004)planes in FIGS. 8 and 9 are compared with each other, both peaks in FIG.8 are extremely high. According to this fact, it is clear that thecrystal grains can be oriented in the c-axis direction by manufacturingthe compact in the magnetic field.

The degree of orientation of the oxide ion conductor 10 can becalculated according to the Lotgering expression as represented by theexpression (1).

$\begin{matrix}{f = \frac{\rho - \rho_{0}}{1 - \rho_{0}}} & (1)\end{matrix}$

In the expression (1), ρ₀ represents the ratio between the totalintensity of the peaks-appeared in the range of the diffraction angle(2θ) of 20° to 60° and the intensity of both peaks on the (002) and(004) planes in relation to the oxide ion conductor produced in nomagnetic field, which is determined by the expression (2).

$\begin{matrix}{\rho_{0} = \frac{\sum\;{I_{0}\left( {00l} \right)}}{\sum\;{I_{0}({hkl})}}} & (2)\end{matrix}$

In the expression (2), ΣI₀(hkl) represents the total intensity of thepeaks appeared in the range of 20° to 60°, and ΣI₀(001) represents theintensity of both peaks based on the (002) and (004) planes.

On the other hand, ρ in the expression (1) represents the ratio betweenthe total intensity of the peaks appeared in the range of thediffraction angle (2θ) of 20° to 60° and the intensity of both peaks onthe (002) and (004) planes in relation to the oxide ion conductor 10produced in the magnetic field, which is determined by the expression(3).

$\begin{matrix}{\rho = \frac{\sum\;{I\left( {00l} \right)}}{\sum\;{I({hkl})}}} & (3)\end{matrix}$

In the expression (3), ΣI(hkl) and ΣI(001) represent the total intensityof the peaks appeared in the range of 20° to 60° and the intensity ofboth peaks on the (002) and (004) planes in the same manner as in theexpression (2).

For example, when ΣI₀(hkl) and ΣI₀(001) are determined with reference toFIG. 9, ρ₀ is 0.057. Similarly, when ΣI(hkl) and ΣI(001) are determinedaccording to FIG. 8, ρ is 0.452. When the variables in expression (1)are substituted with these values, the degree of orientation f of theoxide ion conductor 10 (La₁₀Si₆O₂₇) having the X-ray diffractionmeasurement profile shown in FIG. 8 is 41.9%.

As described above, according to the production method of the embodimentof the present invention, it is possible to obtain the oxide ionconductor 10 in which the c-axis of the crystal grains is approximatelyoriented in the specified direction. Therefore, the anisotropy arises inthe oxide ion conductivity in the oxide ion conductor 10. The extremelyexcellent oxide ion conductivity is shown in the direction of the arrowB shown in FIG. 1.

Further, the oxide ion conductor 10 obtained as described above has theapatite type structure in which the crystal belongs to the hexagonalsystem and the space is expressed as P6₃/m. Therefore, the excellentoxide ion conductivity is exhibited.

This fact is clearly understood from FIG. 10 which indicates thecomparison between the ion conductivity of the oxide ion conductor 10 inthe c-axis direction according to the embodiment of the presentinvention having the X-ray diffraction measurement profile as shown inFIG. 8 and the respective ion conductivities of the conventional oxideion conductor as shown in FIG. 9 and yttria-stabilized zirconia (YSZ).That is, it is clearly understood from FIG. 10 that the oxide ionconductor 10 according to the embodiment of the present inventionexhibits the excellent ion conductivity over the entire temperatureregion.

The embodiment of the present invention has been explained asexemplified by the case in which the crystal grains are oriented in thec-axis direction along lines of magnetic force. However, the crystalorientation is not specifically limited thereto. The crystal grains maybe oriented in any direction irrespectively of lines of magnetic forceprovided that the anisotropy arises in the oxide ion conductivity.

In the embodiment described above, the mixed powder is obtained bymixing the La₂O₃ powder and the SiO₂ powder. However, a mixed powder maybe obtained by using powders of substances other than the oxide, forexample, such that a carbonate salt of lanthanum and a carbonate salt ofsilicon are mixed with each other. It is also a matter of course in thiscase that the ratio of the respective powders should be adjusted so thatLa_(X)Si₆O_(1.5X+12), in which X is not less than 8 and not more than10.

Other rare earth elements and/or trivalent elements may be used in placeof La. Part of the elements A may be replaced with bivalent or univalentelements. Ge or other tetravalent elements may be used in place of Si.In the case of Ge, the sintering temperature can be low, i.e., about1500° C. as compared with Si.

The oxide ion conductor of the present invention is not limited to aconductor having the apatite type structure. It is also possible to usean oxide ion conductor having in the crystal thereof a conduction planeor a conduction direction in which the oxide ion is movable, and showinganisotropy in relation to the oxide ion conductivity. For example,layered perovskite compounds or a series of oxide ion conductorsreferred to as “BIMEVOX” having a basic composition of Bi₄V₂O₁₁ can beused.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A method of producing an oxide ion conductor comprising a compositeoxide which has constituent elements of a trivalent element A, atetravalent element B, and oxygen O, said composite oxide has acomposition formula represented by A_(X)B₆O_(1.5X+12) provided that8≦X≦10, said composite oxide has an apatite type crystalline structure,said oxide ion conductor has anisotropy in oxide ion conductivity, saidmethod of producing said oxide ion conductor comprising: a first step ofmixing a powder of a substance having constituent elements of saidelement A and oxygen O with a powder of a substance having constituentelements of said element B and oxygen O in a ratio to produceA_(X)B₆O_(1.5X+12) provided that 8≦X≦10, for obtaining a mixed powder; asecond step of heating said mixed powder for preparing said compositeoxide having said composition formula represented by A_(X)B₆O_(1.5X+12)provided that 8≦X≦10; a third step of adding said composite oxide to asolvent for preparing a slurry, and then solidifying said slurry in amagnetic field for preparing a compact; and a fourth step of sinteringsaid compact for preparing said oxide ion conductor of said compositeoxide.
 2. The method of producing said oxide ion conductor according toclaim 1, wherein a temperature of heating in said second step is 700° to1200° C.
 3. The method of producing said oxide ion conductor accordingto claim 1, wherein a temperature of sintering in said fourth step is1400° to 1800° C.
 4. The method of producing said oxide ion conductoraccording to claim 1, wherein a rare earth compound is used as saidsubstance which has said element A and oxygen O.
 5. The method ofproducing said oxide ion conductor according to claim 4, wherein alanthanum compound is used as said rare earth compound.
 6. The method ofproducing said oxide ion conductor according to claim 4, wherein one ofsilicon oxide and germanium oxide is used as said substance which hassaid element B and oxygen O.
 7. The method of producing said oxide ionconductor according to claim 4, wherein said rare earth compound is usedso that X=9.33.